This application pertains to the art of electronic phase shifters and, more particularly, to digitally controlled phase shifters.
The most commonly used shifter and the most easily understood is the time shifter used in antenna-beam-forming networks. When a plurality of antennas are placed in a linear array, a radio frequency (RF) signal may reach each of the antennas in the array at a different time. That is to say, an RF wave front arriving at right angles to the line of the array would excite all antennas together, and the signals in the antenna feeds would be in phase and ready to be added as such at a receiver installation. On the other hand, a wave front arriving at a significant angle would result in different phases at the antenna outputs. Judicious placement of delay lines, in proportion to the delays encountered at the antenna itself permits the signals in the feed lines to be re-aligned in phase and time. Adjustment of the feed line delay values permits the array to be steered as if the physical structure itself were rotated.
Application of a true phase shifter is a more subtle matter. One example is a receiver-combiner system, in which multiple diversity inputs are presented to circuits that must combine them coherently in both the time and the frequency domains. If the incoming signals are merely delayed in time, as in the antenna array above, a simple time shifter could be used to align the signals in phase and in time. However, the incoming signals may have been delayed in a more complex manner. For example, two separate signals, each composed of a burst of RF, may have the same starting time but after allowing a time to pass and permitting transient effects to die out, the steady-state RF phase within the otherwise identical envelopes varies from one signal to the other. If a time shifter were used to attempt to remove the difference, the delay introduced would shift the pulse envelope as well. In that this would not be desirable, the RF phase of the signal should be changed without varying the envelope by using a true phase shifter, which can vary the phase without disturbing the time delay.
A typical phase shifter comprises a quadrature hybrid circuit having one lead thereof as the phase shifter input, an output isolated from the input and two quadrature hybrid inputs connected to ground through separate, reactive components such as capacitors or inductors.
Performance in such phase shifters is usually acceptable even though there is some frequency dependence on the resultant phase shift. Such frequency dependence is much less than that of the time shifter in which the shift is directly proportional to frequency, but such dependence is unsatisfactory for critical applications.
The invention is particularly applicable to phase shifting with improved frequency independence of a phase shift and will be described with particular reference thereto. Although it will be appreciated that the invention has broader applications such as in high precision, beam-forming networks, and other digitally controlled phase shifters.